Metal cations belonging to the first transition series are known to play important coenzymatic roles in metabolism. Zinc is known to be a coenzyme for over eighty different enzyme systems including those directly involved with DNA and RNA synthesis such as thymidine kinase, DNA and RNA polymerases, reverse transcriptase and terminal deoxynucleotide transferase. Among its other coenzyme functions, iron is the coenzyme for myoglobin, the cytochromes and catalases and is thus essential for oxidative metabolism. Manganese and copper also play significant coenzyme roles, and other metal cations in the first transition series are considered to be essential trace elements although their metabolic role is less well defined.
Compounds capable of forming complexes with metal cations, which compounds are commonly referred to as chelators or ligands, are known to have a variety of uses in medicine. These include their use as pharmaceuticals in treating heavy metal poisoning, in treating diseases associated with trace metal excess such as iron storage and copper storage diseases (hemosiderosis and Wilson's disease, respectively), as radiopharmaceutical agents in nuclear medical imaging when forming complexes with radioactive metals, as contrast enhancement agents in magnetic resonance imaging (MRI) when forming complexes with paramagnetic metals, and as contrast enhancement agents in radiography when forming complexes with heavy metals.
Examples of ligands employed in treating heavy metal poisoning such as that due to lead, mercury, and other metals, are ethylenediamine tetraacetic acid (EDTA) and diethylenetriamine pentaacetic acid (DTPA). The ligand desferrioxamine is used in treating iron storage disease, and the ligand penicillamine is used in the mobilization of copper in the treatment of Wilson's disease.
Examples of complexes used to form radiopharmaceuticals useful in evaluations of the kidneys, bone and liver are complexes of technetium-99m (.sup.99m Tc) with diethylenetriamine pentaacetic acid (DTPA), dimercaptosuccinic acid (DMSA), methylene diphosphonate (MDP) and derivatives of iminodiacetic acid (IDA).
Complexes of paramagnetic metal cations which are useful as MRI contrast agents operate by accelerating proton relaxation rates. Most commonly a metal cation, such as gadolinium (III), having a large number of unpaired electrons is complexed by a ligand suitable for complexation of that cation. An example is gadolinium (III) complexed by DTPA.
Chelators with affinity for iron cations have been shown to inhibit cell proliferation. Desferrioxamine is one example of such a chelator. This effect is thought to be a consequence of the complexation of tissue iron by the chelator, which thereby deprives the proliferating cells of a source of iron for critical enzyme synthesis. Moreover, it is believed that certain types of tissue damage are mediated by the formation of free radicals. It is also appreciated that catalytically active iron catalyzes formation of the highly active hydroxyl free radical. Based on such relationships chelators (ligands) for iron such as desferrioxamine and the experimental iron chelator "L1" (1,2-dimethyl-3-hydroxypyrid-4-one) have been examined in management of conditions where free radical mediated tissue damage is believed to play a role, as well as in clinical management of conditions in which control of cell proliferation is desired. Conditions where the administration of iron chelators has been evaluated include: rheumatoid arthritis, anthracycline cardiac poisoning, reperfusion injury, solid tumors, hematologic cancers, malaria, renal failure, Alzheimer's disease, myelofibrosis, multiple sclerosis, drug-induced lung injury, graft versus host disease, and transplant rejection and preservation (Voest, E. E., et al., "Iron Chelating Agents in Non-Iron Overload conditions," Annals of Internal Medicine 120(6): 490-499 (15 March 1994)).
Agents which inhibit cell replication have found use in the prior art as chemotherapeutic agents for treatment of neoplasia and infectious disease, for suppression of the immune response, and for termination of pregnancy.
Such agents usually act by inhibiting DNA, RNA or protein synthesis. This results in a greater adverse effect on rapidly proliferating cell populations than on cells "resting" in interphase or proliferating less rapidly.
Such agents may possess a degree of selectivity in treating the rapidly proliferating offending cell population, particularly in the case of certain neoplasias and infectious processes. These agents also inhibit replication of normal cells of the host organism, to varying degrees. Cells of the immune system proliferating in response to antigenic challenge are sensitive to such agents, and accordingly these agents are useful in suppressing the immunological response. Examples are the suppression of the homograft rejection response following tissue transplantation and the treatment of autoimmune disorders. Replication of protozoan, bacterial and mycotic microorganisms are also sensitive to such agents, which makes the agents useful in treating infections by such microorganisms.
Agents which suppress cell replication by inhibiting DNA or RNA synthesis have primarily found utility in treatment of neoplastic diseases. The glutamine antagonists azaserine, DON, and the anti-purines such as 6-mercaptopurine and 6-thioguanine principally inhibit DNA synthesis by their action on phosphoribosylpyrophosphate amidotransferase, the enzyme involved in the first step in purine nucleotide synthesis. The folic acid antagonists aminopterin and methotrexate inhibit DNA synthesis (and other synthetic processes involving one carbon transport) by inhibiting the dihydrofolate reductase enzyme system, thereby interfering with formation of tetrahydrofolate, which is necessary in transfer of one-carbon fragments to purine and pyrimidine rings. Hydroxyurea inhibits DNA synthesis by inhibiting ribonuclease reductase, thereby preventing reduction of ribonucleotides to their corresponding deoxyribonucleotides. The anti-pyrimidines such as 5-fluorouracil inhibit DNA synthesis by inhibiting thymidylate synthetase. 5-Fluorouracil may also be incorporated into fraudulent RNA molecules. Bleomycin appears to inhibit DNA synthesis by blocking thymidine incorporation into DNA, although it may have other mechanisms of action. Agents such as 5-bromouracil and iododeoxyuridine may be incorporated into DNA in place of thymidine, and cytosine arabinoside may be incorporated into DNA in place of 2'-deoxycytidine. The fraudulent DNA produced by these incorporations interferes with the information transmittal system for DNA.fwdarw.RNA.fwdarw.protein synthesis.
Alkylating agents used in treating neoplasias, such as the nitrogen mustards, ethylene imines, alkyl sulphonates and antibiotics such as mitomycin C, suppress cell replication by attacking DNA and forming covalent alkylate linkages within preformed DNA, thereby interfering with DNA function and replication. The activity of such agents is therefore not limited to inhibition of cell replication alone.
Agents used in treatment of neoplasias such as 8-azaguanidine and 5-fluorouracil inhibit cell replication by being incorporated into fraudulent RNA. Agents such as actinomycin D, daunorubicin, nogalomycin, mithramycin and adriamycin are thought to inhibit RNA polymerase by strongly binding to DNA and thereby inhibiting DNA to RNA transcription.
Certain agents which inhibit cell replication by arresting metaphase (examples of such agents are colchicine, vinblastine, vincristine, podophyllotoxin, and griseofulvin) or arresting telophase (cytochalasins) have also been shown to be active in treating neoplasias or microbial infections.
Agents which inhibit cell replication primarily by inhibition of protein synthesis have found utility in the treatment of microbial infections. The tetracyclines, streptomycins and neomycin, for example, inhibit protein synthesis by inhibition of the mRNA-ribosome-tRNA complex. Chloramphenicol, erythromycin, lincomycin, puromycin appear to inhibit protein synthesis by inhibition of the peptidyl synthetase reaction. A miscellaneous group of antibiotics appear to act by inhibition of translocation of the ribosome along mRNA. Penicillins act by inhibiting synthesis of the bacterial cell wall.
Complexes of heavy metals such as platinum have been found to be active in treatment of certain neoplasias. The inhibition of cell replication by these complexes is attributed to the in vivo hydrolysis of one or more of the coordinating ligand sites occupying positions in the coordination shell of the metal. This hydrolysis liberates the coordination sites for in vivo interaction with nucleophilic donor sites which are critical to replication or survival of the cell population. There is a wide diversity of such donor sites in vivo, but it is believed that one critical set of donor sites involves binding of the platinum to two guanine or one guanine and one adenine residue of opposing strands of DNA.
The mechanisms of action of the various agents employed in treating protozoan infection are largely unknown. However, it has been demonstrated that agents which interfere with cell replication can be active in treating such infections. For example, the antimalarial agent chloroguanide and the diaminopyrimidines act as selective inhibitors of plasmodial dihydrofolate reductase thereby inhibiting plasmodial DNA replication. Tetracyclines possess antimalarial and antiprotozoal activities possibly acting by mechanisms similar to those operative in their inhibition of bacterial replication. The antibiotics puromycin and erythromycin, as well as tetracyclines which inhibit microbial replication, have also been employed in treatment of amebiasis. The antiprotozoal effects of the diamidines is believed to be due to their inhibition of cell replication by interference with DNA.
Based on what is known from the action of the agents cited above, one can readily conclude that agents which inhibit cell replication, regardless of the specific biochemical mechanism involved, have utility in the treatment of a wide variety of neoplastic and infectious diseases and in the management of certain of the body's responses which are mediated through selective in vivo cell replication.